Mems electrostatic actuator

ABSTRACT

A MEMS electrostatic actuator includes a bottom plate affixed to a substrate and a top plate suspended above the bottom plate. The top plate has a parallel plate center section and two rotating members electrically connected to the center section. Each rotating member is attached centrally of the rotating member for rotation about an axis of rotation to a set of anchor posts. The attachment includes at least one pair of torsional springs attached along each axis, each spring comprising a rectangular metal square that twists as the rotational members rotate. Electrostatic pull-down electrodes are underneath each rotational member.

This application claims the benefit of U.S. Provisional Application No.61/534,157 filed Sep. 13, 2011, and U.S. Provisional Application No.61/606,041 filed Mar. 2, 2012, the entireties of both of which arehereby incorporated herein by reference.

A MEMS electrostatic actuator and methods for its actuation aredescribed.

BACKGROUND

As shown schematically in FIG. 1, a MEMS electrostatic actuator 10 (alsoknown as a capacitive actuator) generally comprises two parallelelectrodes, one electrode (the “bottom plate”) 12 of which is typicallyfixed to a substrate 14 and the other electrode (the “top plate”) 16 ofwhich is suspended with a spring suspension 18 spaced by an air gap 20above the bottom plate. A thin solid dielectric film 22 is deposited onthe surface of the bottom plate 12 to prevent shorting. A voltage isapplied between the two electrodes, creating an electric field thatproduces an attractive force between the two electrodes 12, 16. Thisforce causes displacement of the top plate 16, until the top plate 16makes contact with the dielectric 22 on the bottom plate 12. Once thevoltage is removed, the top plate 16 returns to its original positionunder the restorative force of spring suspension 18.

A parallel plate actuator with a torsional architecture is described inP. Farnelli, et al., “A Wide Tuning Range MEMS Varactor Based on aToggle Push-Pull Mechanism,” Microwave Integrated Circuit Conference,474-477 (2008), and in F. Solazzi, et al., “Active Recovering Mechanismfor High Performance RF MEMS Redundancy Switches,” Proceedings of the40^(th) European Microwave Conference, 93-96 (September 2010), both ofwhich are hereby incorporated herein by reference.

One problem that arises in connection with MEMS capacitive actuators isstiction—static friction associated with adhesion of contactingsurfaces. Stiction occurs when Van der Waals forces and the like causethe top plate to stick down on the dielectric on the bottom plate, evenafter the actuation voltage is removed.

A second problem arises when the top plate is landed on the bottomplate. At this point, the electric field across the thin soliddielectric that separates the two plates is very high. This causeselectrostatic charge to be injected into the dielectric, a phenomenonknown as dielectric charging. The charged dielectric has its owncontribution to the electric field between the two plates, and it cancause undesirable effects: It can change the release voltage, thevoltage at which the top plate returns to its original position. It canchange the land voltage, the voltage at which the top plate lands on thebottom plate. It can shift the voltage that produces minimum capacitanceaway from OV. These issues are problems for repeatability andreliability, and they can cause early device failure.

A third problem that arises when the device is used as a variablecapacitor for RF applications is self-actuation. When RF energy isapplied to the plates, the RMS voltage of the RF signal can cause thetop plate to move and potentially even land on the bottom plate, even ifthere is no DC actuation voltage present. This is especially a problemin high power RF applications, as it limits the power-handlingspecification for the device.

A fourth problem, similar to self-actuation, is self-latching. In RFapplications, when the top plate is landed on the bottom plate, the RMSvoltage on the RF signal on the bottom plate can hold the top plate downand prevent it from releasing, even when the DC actuation voltage isremoved. This effectively prevents hot-switching, and presents asignificant problem if the device is used in CDMA systems. Theself-latching voltage is usually much lower than the self-actuationvoltage.

A fifth problem is related to the flatness of the suspended top plate.Since the device is a parallel plate capacitor, both plates shouldideally be flat and parallel. However, residual stresses frommanufacturing can cause the top plate to curl. When the top plate islanded on the bottom plate, this curling creates an air gap between thetwo plates, greatly reducing on-state capacitance. This is illustratedin FIG. 2 which shows top plate 16 landed on solid dielectric layer 22above bottom plate 12 on substrate 14. Top plate 16 is curled downwardmaking contact peripherally with dielectric 22 over bottom plate 12 andleaving an air gap 20′ centrally between the regions of contact.

Other problems are slow switching speed and slow actuator settling timeupon release. When the DC actuation voltage is removed, the top platemoves upward due to the restoring force of its spring suspension.Generally, this mass-spring system is underdamped, causing oscillation.The problem arises when the settling time for the oscillation is longerthan the maximum switching time of the system, for example intransmit/receive switching application for GSM mobile handsets.

Another problem is one of system integration. In a typical RF handsetapplication, there is no high voltage power supply available. Adding anoff-chip supply is highly undesirable due to cost and board real estate.Therefore, the DC actuation voltage must be supplied by on-chipcircuitry. It is a manufacturing challenge to co-locate MEMS and CMOScomponents. RF CMOS processes may provide good RF performance, but maybe cost prohibitive. Also, they may not have the right component setrequired to generate an on-chip electrostatic supply. On the other hand,standard mixed signal processes provide poor RF performance. Moreover,there is a limit to the magnitude of the voltage that can be generatedon the chip. As such, the actuation voltage of the MEMS structure mustbe below that limit.

SUMMARY

A MEMS electrostatic actuator and methods for its actuation aredescribed that address the above-identified problems.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments are described with reference to accompanyingdrawings, wherein:

FIG. 1 is a schematic representation of a parallel plate MEMSelectrostatic actuator;

FIG. 2 is a schematic representation illustrating the problem ofcurling;

FIGS. 3-20 illustrate example embodiments.

DESCRIPTION OF EXAMPLE EMBODIMENTS

An example MEMS structure 100 including a plurality of electrostaticactuators 102 is illustrated in FIG. 2. As shown in FIGS. 4A-4C, eachactuator 102 has a first capacitor plate section 104 (the top plate)that is suspended above and a second capacitor plate section 106 (thebottom plate) that is affixed to a substrate 108. The two sections 104,106 are separated by an air gap 110, and the suspended section issupported by anchor vias 112.

To address the problem of dielectric charging, the embodiment uses atorsional actuation. An articulated structure of a type suitable fortorsional actuation is illustrated in commonly owned U.S. Pat. No.8,171,804, incorporated herein by reference. The suspended portion 104of the actuator is split up into three sections as illustrated in FIGS.5-8: a parallel plate center section 114, and two rotating paddle-shapeddrive members 116. All three sections 114, 116 are electrically shortedto each other.

Each drive member 116 has anchor portions 118 attached to the substratethrough a set of four anchor posts 112 at positions 120 (see FIG. 7)centrally of the drive member 116. The anchor members 118 connect toremaining portions of the drive members through a torsion hinge member122 which provides a horizontal axis of rotation 124 about which thedrive member can pivot relative to the substrate 108. The drive members116 are disposed so that their respective axes 124 are parallel.

The main portion 126 of each drive member 116 is formed in a metal layerand including corrugations or raised portions 128 which provide rigidityto an otherwise generally planar, flat teeter-totter structure whichpivots about the axis 124. The main portion has inner and outer ends130, 132 laterally dimensioned with outer ends 132 having a smallerwidth dimension (direction parallel to axis 124) that inner ends 132,allowing like ends of drive members 116 of neighboring actuators 102 tobe brought into interdigitated positions that enable staggered closerpacking of pluralities of such structures (see FIG. 3). The wider innerends 132 match a generally uniform width dimension of a correspondingplate section 114.

The plate section 114 has a main section 136 of generally rectangularshape. It, too, is generally planar with the addition of undulations(corrugations or recesses) to impart rigidity (see FIG. 8). A width-wiseextending pivot pin member 140 is provided at opposite ends of the mainsection 136, traversing longitudinally directed tab extensions 142formed at opposite ends of the main section 136. Like tab extensions 144(FIG. 7) formed at adjacent ends of the drive members 116 are disposedto extend into respective openings 146 between adjacent tab extensions142 and contact the member 140, allowing relative pivotal motion of themembers 114, 116 about horizontal axes 148—parallel with the axes124—established by the members 140. Openings 152 formed through the mainsection 136 provide release holes to prevent stiction when the mainsection 136 is in the actuated downward position.

The depicted actuator 102 has four parallel horizontal axes of rotation124, 148 enabling vertical movement of the main section 136 whilekeeping the planar section generally horizontal. As illustrated in FIGS.6A-6C, one to two pairs of torsional springs are positioned along theaxes, each consisting of a thin rectangular metal square that twists asthe rotational members rotate.

As shown in FIGS. 4A-4C, electrostatic pull-in electrodes (i.e. driveelectrodes) DRVIN are positioned on substrate 108 below inner portions130 of drive members 116 between vias 112 and hinge members 122.Electrostatic pull-out electrodes DRVOUT are positioned on substrate 108below outer portions 132 of drive members 116 between vias 112 and theoutermost portions (away from plate section 114) of drive members 116.The electrostatic electrodes act in pairs, with the inner set (DRVIN)acting together and the outer set (DRVOUT) acting together. When anelectric potential difference is placed between the inner set of driveelectrodes and the top plate, an electrostatic force is exerted on thetop plate. This causes the two drive members 116 to rotate inwards abouttheir respective axis 124 (toward the plate section 114), which in turncauses the parallel center plate 114 to move downwards while remainingparallel to substrate 108. As the potential difference increases, therewill be a point at which the electrostatic force pulling the structuredown becomes greater than the spring restoring force provided by thetorsional springs. At this point, the top plate collapses down onto thedielectric layer formed over the bottom plate 106. This is calledpull-in and represents movement into the position shown in FIG. 4C.

In accordance with an aspect of the invention, the inner edge of eachrotating member 116 includes thin rectangular protrusions known as“spring tips” extending longitudinally, laterally outward of andgenerally parallel with the tab extensions 144. These spring tips extendinwardly beyond the inner edge of the rotating member. When pull-inoccurs, the first parts of the suspended section to touch elementsdisposed on substrate 108 are the spring tips. These touch the GNDelectrodes shown in FIG. 4C which are located below the pivot pins 140.The center plate 114 then lands on a thin dielectric layer covering thebottom plate 106 located below it. Plate 106 is illustrated by thesignal electrode SIG shown in FIGS. 4A-4C. Plate 106 (i.e., SIGelectrode) is covered with the thin dielectric layer to preventelectrical shorting. In the inwardly downward movement of drive members116, after the spring tips have made contact, hard stops in the form ofrigid protrusions (hard stops) on the inner edges of the rotating drivemembers 116 next touch down on the GND electrodes, thereby preventingfurther rotation.

No portion of the rotating beams actually touches the biased driveelectrodes. Since there is no direct electric contact and adequate airgap space between the drive electrodes and the three top plate segments,the field across the surface dielectric is greatly reduced. This greatlyreduces oxide charging.

Furthermore, the device is designed to be bi-stable. The top plate caneither be “pulled-in” or “pulled-out”. In pull-in, the DRVIN electrodesare biased, causing the rotational beams to rotate inward, and causingthe parallel center beam to move downward. In pullout, the bias isremoved from the DRVIN electrodes and applied to the DRVOUT electrodes.This causes the rotational beams to rotate outward, and the parallelcenter beam moves upward. There are hard stops in both the pull-in andpull-out cases, preventing over rotation and providing two mechanicallystable states.

The top plate will return to a flat state (FIG. 4A) if bias is removedfrom all DRV electrodes. However, for the illustrated configuration theflat state may not be mechanically stable and achieving a flat state (nobiasing) position may be influenced by residual charge in the surfacedielectric, on the memory of the springs, and other phenomena. Further,in the absence of hard stops, the flat state may have a long settlingtime before reaching a steady state. For these reasons, the flat stateis not utilized as a neutral position in operation of the illustrateddevice. (Of course, additional electrodes or other accommodation may bemade if utilization of the flat state is desired.)

Switching directly between the pull-in and pull-out state solves severalof the problems outlined above. First, it leads to fast switchingspeeds. Mechanical hard stops in both the up and down state mean thatmechanical ringing is quickly damped out through the substrate. Thedevice settles quickly and reaches a steady state much faster than itwould if the flat state were used. Furthermore, since the device iselectrostatically held in the up state, small amounts of residual chargein the dielectric on the DRVIN electrodes will not have an effect on theposition or stability of the up state. Also, the pull-out state greatlyincreases self-actuation voltage. Since the device is actively held inthe up state, it takes much more voltage on the SIG electrode to disturbthe top plate when it is pulled out. Finally, the pull-out featurecauses the parallel portion of the top plate to be pulled further awayfrom the SIG electrode than it would otherwise be in the flat state.Since it is further away, any voltage on the SIG electrodes exerts lessforce on the top plate (since force is proportional to the inversesquare of distance), further increasing the self-actuation voltage. Thecapacitance between the SIG electrode and the top plate in the up stateis also reduced.

The stiction problem is addressed by several design features. First,stiction is a function of the area in which the top and bottom platescome into intimate contact. The surface of the bottom electrode is madevery rough, with a peak-to-valley surface roughness around 800 A. Thisroughness is achieved by making the bottom electrode 3 um thick,allowing large grains to form in the metal. These grains produce surfaceroughness. The roughness limits the amount of area in which the top andbottom plates come into intimate contact, thereby reducing stiction.

A second design feature for stiction reduction is the inclusion ofnotches in the edges of the top and bottom plates. Stiction forces tendto be stronger along edges than along main surfaces. Thus, even if amain surface of the top plate releases off the bottom plate, one of theedges might still stick, causing the parallel center plate to becometorqued at an angle. Adding small notched protrusions to the edges ofthe top and bottom plates greatly reduces the length of edge that couldpotentially stick.

A third design feature that helps with stiction is the addition of thespring tips, already mentioned. As these spring tips land, they deflectand thereby store energy. When the pull-in voltage bias is removed, thespring tips release the stored energy, helping the top plate to pop offthe surface of the bottom plate. It is important to note that the forcedue to the pull-in voltage bias is much stronger when the two plates areclose together compared to when they are far apart. Since the springtips do not make contact and do not start deflecting until the top andbottom plates are brought close together, they can take advantage of theadditional electrostatic force to store an extra “kick” of energy. Thisis similar to the energy storage capability of spring tips used toprovide a restorative force in the deflection of micromirrors used indigital micromirror devices (DMDs).

A fourth design feature that helps with stiction is the use of torsionalsprings having high restoring force. Compared to an SPD DMD micromirror,the torsional springs in the described capacitive actuator are muchshorter but with a similar thickness, leading to higher restoring force.There are also 4 to 8 pairs of torsional springs (one to two pairs alongeach of the four axes of rotation), compared to the single pair in a DMDmicromirror. The total restoring provided by the torsional spring andspring tips in an implementation of the described actuator is 3-5 uN.

One of the consequences of high restoring force is that it may make thetop plate difficult to pull down. The electrostatic pull-in force musttypically be greater than the restoring force. The need for largepull-in force can be met by increasing the pull-in bias voltage, but itmay be desirable to keep the bias voltage low so that it can be drivenby on-chip CMOS circuitry. Therefore, increased pull-in force mayinstead be accomplished through use of large pull-down electrodes. Sincethe outer rotation axis acts as a fulcrum point, the further thepull-down electrode extends laterally away from the fulcrum, the largerthe pull-down force will be. This is due to the multiplication ofmechanical force provided by the lever that is formed by the fulcrum.

To take full advantage of the lever effect, it may be necessary to makethe actuator very long. Long levers produce greater leverage than shortlevers. Long actuators may face two main problems, however: one iscurling of the suspended top plate due to residual manufacturingstresses, and the other is reduced capacitance density since such alarge area is occupied by the pull-down electrodes instead of the signalelectrodes. The second problem is partially alleviated by making theouter portion of the actuator narrow, so that they the outer portionscan be staggered and interleaved with adjacent actuators. This allowslonger actuators with a higher overall capacitance density for thearray.

Also, a low pull-down voltages (20V-30V) of the actuator is madepossible because the torsional hinges are very thin but are manufacturedwith good uniformity across the wafer. The thinness of the hinge metalis accomplished through the use of a superstructure manufacturing flowused elsewhere in connection with DLP™ DMD devices. A DMD devicefabrication process is described in M. Mignardi and R. Howell, “TheFabrication of the Digital Micromirror Device,” Commercialization ofMicrosystems '96 (1996), incorporated herein by reference.

Such manufacturing flow also allows the actuator to be built directly ontop of the CMOS circuitry that drives it. However, standard CMOS isbuilt on low resistivity substrates, which inherently result in high RFloss. This system-integration issue is resolved by including a solidmetal ground shield between the CMOS and the variable capacitor. Theground shield isolates the RF energy from the substrate, so it does notexperience high substrate loss. However, that ground shield addsunwanted parasitic capacitance that can reduce the capacitance ratio. Toprevent this, a stack of thickened metals and thickened dielectrics canbe used to increase physical distance between the variable capacitor andthe ground shield. This extra distance reduces the parasitic capacitanceof the ground shield.

The third and fourth design features that combat stiction also increaseself-latch voltage. This is because both stiction and self-latch resultfrom an attractive force between the top and bottom plates that tries tokeep the top plate from pulling out. It is noted that the electrostaticforce from RF energy on the SIG electrode varies with the inverse of thegap squared. Thus, even through the spring tips release their energyover only a short z-direction (vertical) distance, the self-latchingforce becomes significantly weaker over that distance, allowing thedevice to pull out.

The device can also benefit from the dynamic release of energy by thespring tips. Once the device is in the landed state, the spring tips canbe further depressed and then released by pulsing the bias voltage onDRVIN. If this pulse matches the resonant frequency of the spring tips,the top plate will receive a resonant boost that amplifies the pull-outforce by a factor of two to four times (2×-4×).

There are also design features intended to improve the planarity of thesuspended beam. Firstly, the overall lateral dimensions of the actuatormay be kept small (<100 um). Residual stresses from manufacturing mayadd up over the length of the suspended beam, so a shorter beam may beless prone to stress-curling. Also, since the top plate may be suspendedover 3 um thick surface electrodes, the topology of the surfaceelectrodes may be carried through the sacrificial layer up to the topplate. To prevent this, a planarization process can be employed in whichthe gaps between surface electrodes is filled with extra sacrificialmaterial. That way the top plate can be deposited on a flat surface andavoid taking on the topology of the metal underneath it.

Furthermore, oxide ridges may be embedded into the suspended top plate.These oxide ridges provide added z-direction rigidity, allowing theactuator to resist curling due to residual stresses from manufacturing.

The suspended plate may be formed by first depositing a thin hingematerial on top of a sacrificial layer. A hard oxide mask may then bedeposited and patterned in order to define the torsional hinges andspring tips. A thick beam layer is then deposited and patterned to formthe suspended plates. The pattern etch goes all they way down throughthe hinge layer, and the hard oxide mask protects the hinge from theetch in the areas where the torsional springs and spring tips are to bedefined. The oxide mask is then stripped. However, in order to produceoxide ridges, the hard oxide mask is left embedded within the beam incorrugation patterns designed to give maximum z-direction rigidity. Thisproduces a hinge-oxide-beam sandwich, where the beam completely coversthe oxide ridges and protects them from the oxide strip process. Etchholes are also included in the beam for proper undercut (removal ofsacrificial layer).

Those skilled in the art to which the invention relates will appreciatethat the disclosed example embodiments may be modified and also thatmany other embodiments are possible within the scope of the claimedinvention.

What is claimed is:
 1. A MEMS electrostatic actuator, comprising: abottom plate affixed to a substrate; a top plate suspended above thebottom plate; the top plate comprising a parallel plate center sectionand two rotating members electrically connected to the center section;each rotating member is attached centrally of the rotating member forrotation about an axis of rotation to a set of anchor posts; theattachment including at least one pair of torsional springs attachedalong each axis, each spring comprising a rectangular metal square thattwists as the rotational members rotate; and electrostatic pull-downelectrodes underneath each rotational member.